<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>hello world</title>
    <script>
        function a() {
            document.getElementById("one").style.color = "#ff0000";
        }
        function b() {
            document.getElementById("one").style.color = "#000";
        }
    </script>
    <style>
        div {
            width: 300px;
            height: 180px;
            margin: 0px auto;
            /*div盒子居中*/
            text-align: center;
            /*文字居中*/
            color: #000;
            font-size: 24px;
            background-color: aqua;
        }
    </style>
</head>

<body>
    <div id="one">
        <br>
        <br>
        Hello_world!
        <br>
        <button onclick="a()" style="color: #000" id="b1">变为红色</button>
        <button onclick="b()" style="color:#000" id="b2">复原</button>
    </div>

</body>

</html>